1. Field of the Invention
This invention relates to a process for abrading, shaping and grinding fragile materials prior to sintering or firing in the case of ceramic materials and for sintering spheres by using particles of tungsten oxide as a separating agent.
2. Description of the Previously Published Art
Conventional grinding techniques for producing spheres such as metallic balls of steel, bronze, brass aluminum and the like for use as ball-bearings and similar fabrication, involve operations employing dense, hard materials. Those methods are generally unfit for similar use involving fragile substances such as green ceramics and pressed powder compacts prior to sintering or firing. For those harder substances, the grinding process usually uses two grinding surfaces such as discs and belts, both applied with pressure against the object to be polished or ground. Such a process is used, for example, in U.S. Pat. No. 3,791,083 to Messerschmidt. However, green ceramics and powder compact substances such as ceramics and various alloys such as iron are brittle by comparison and they are easily fractured or crumbled when pressure is applied during a grinding or finishing or shaping operation.
Green ceramics and powder metallurgical compacts in large numbers can most conveniently be formed into a generally spherical shape by compaction in a multiple cavity die shaped to produce spherical shape. Unfortunately, this produces spherical shapes with parting lines or belts which must be removed either prior to firing or sintering or subsequently. After firing or sintering, the removal process is considerably more difficult where the materials are relatively hard compared to the grinding medium. This makes the grinding operation slow and expensive.
It is known to hold the work pieces in a substantially static position in small numbers for polishing and abrading with pressure. U.S. Pat. No. 3,103,770 to Carter discloses a technique for shaping a single crystalline sphere at a time or at best a limited number. This is done by a vertical tube into which is dropped a single work piece at a time and requires pressure to be placed downwardly by a weight to hold the work piece against the grinding surface. It is acknowledged that the machine output is limited, but can be increased somewhat only by increasing the number of holding devices. However, it is still providing each vertical tube with only a single sphere at a time which is extremely limited. Except for limited movement of the sphere against the grinding surface, the sphere is basically stationary in the vertical tube.
U.S. Pat. No. 4,903,439 to Hoffman discloses a vertically-oriented tube which also provides for polishing and grinding into a sphere a single rough-shaped object at a time which is held in substantially the same place.
There clearly is a need for a process and apparatus which avoids the use of a pressure device downwardly forcing the work piece against a grinding surface and which provides for a large number of work pieces to be rapidly, safely and efficiently ground and processed at one time in a uniform manner.
There is also a need for an expensive separating agent to keep spheres apart when they are being sintered in a hydrogen containing atmosphere.
3. Objects of the Invention
It is an object of this invention to provide an improved technique for simultaneously grinding and abrading a large number of roughly formed work pieces to form true spherical shaped objects.
It is another object to provide techniques for forming true spherical-shaped objects in large quantities of green ceramic articles and pressed powder metallurgical compacts.
It is another object of this invention to provide a production process for rapidly and economically shaping large numbers of spheres of green ceramics and powdered metals such as iron alloys like Fe and W of varying weight/weight percentages.
It is another object of this invention to provide a new process of utilizing the surfaces of the spheres themselves by their dynamic motion to grind a smooth spherical surface.
It is another object of this invention to provide a process by which a large number of work pieces are simultaneously rapidly, safely and efficiently ground into spherical shapes in a substantially uniform manner.
It is a further object of the invention to provide a sphere grinding apparatus where the work pieces progressively move through a channel in a plate while being abraded and moved along the channel by a movable abrasive surface, to thereby continuously form the final smooth spheres.
It is a further object of the invention to provide a sphere grinding apparatus where as the work pieces are moved through a channel in a plate to be abraded by a movable abrasive surface, as well as vibration of the channel. The work pieces are not under pressure as they contact the abrasive surface.
It is yet a further object of the present invention to provide a process whereby pressed powdered metal compacts are rendered spherical and subsequently sintered in an atmosphere containing hydrogen, in the presence of powdered or particulate metal oxide or metal oxides or mixtures of metal oxides to produce sintered metal spheres and particulate metal suitable for further use in making the powdered metal compacts or other forms of the metal made from the reduced metal oxides.
It is a further object of the invention to reduce fusion of the separate spheres to adjacent spheres during sintering at high temperatures in a hydrogen atmosphere by applying tungsten oxide particles to the surface of the spheres prior to sintering.
These and further objects of the invention will become apparent as the description of the invention proceeds.